I. Field of the Invention
This invention relates generally to the field of stereolithography, and more specifically, to an apparatus and related method for rapidly providing a flat working surface of photocurable resin for the formation of a layer of a multi-layer stereolithographic part.
II. Background
Stereolithography is a process for building up a reproduction of an object layer by layer such that the layers are sequentially formed and adhered to adjacent layers until the overall reproduction is complete. The stereolithographic reproduction is commonly referred to as a stereolithographic part, or more simply, part. This process is described in more detail in U.S. Pat. No. 4,575,330, entitled "APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY," by Charles W. Hull, which issued Mar. 11, 1986, and which is hereby fully incorporated by reference herein as though set forth in full. As described in that patent, a stereolithographic apparatus ("SLA") is an apparatus for producing an object through the process of stereolithography. Furthermore as described in that patent, stereolithography is a method and apparatus for making solid objects by successively "printing thin layers of a curable material, e.g., a UV curable material, one on top of the other. An embodiment of an SLA comprises synergistic stimulating means such as a UV laser or the like, photocurable liquid resin, and elevator means. This embodiment of the SLA forms each layer of the part by tracing each layer on a surface of the resin typically referred to as the designated working surface, or more simply, working surface. The layer is traced at an intensity and time sufficient to cure the liquid resin to a predetermined thickness extending beyond the designated working surface.
For further details of stereolithography, reference is made to U.S. Pat. No. 4,575,330 the following U.S. patents and pending U.S. and patent applications which are incorporated herein by reference in their entirety, including appendices attached thereto or material incorporated therein by reference, as if fully set forth:
U.S. Pat. No. 5,104,592;
U.S. patent application Ser. No. 331,644, filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY";
U.S. Pat. No. 5,015,424;
U.S. Pat. No. 4,999,143;
U.S. Pat. No. 5,076,974;
U.S. Pat. No. 5,058,988;
U.S. Pat. No. 5,059,021;
U.S. Pat. No. 5,123,734;
U.S. patent application Ser. No. 365,444, filed Jun. 12, 1989, entitled "INTEGRATED STEREOLITHOGRAPHY".
The part is built on and supported by a platform, which is integral with the elevator means as it is being built up. The elevator means can either raise or lower the platform. Typically, the first layer does not adhere directly to the platform, but instead adheres to supports, which are built up from curing the liquid resin, and which in turn, adhere to the platform.
Subsequently formed layers are then cured such that they adhere to a preceding layer while they are being cured. As the part is being built up, the elevator means progressively moves the already cured layers away from the designated working surface. At each step throughout the build-up of the part, after a layer has been formed, the elevator means moves that layer (along with all the other formed layers) away from the working surface so that fresh liquid resin, which will be used to form the next layer, flows over the previous layer. This step, whereby the liquid resin is caused to flow over the previous layer, is typically known as the recoating step.
Typically, the next layer will have a predetermined thickness, and to speed up the flow of fresh resin over the previous layer, the elevator means will move the just-cured layer away from the working surface by more than the predetermined thickness. If the previous layer were only moved by the predetermined thickness, the surface tension and viscosity of the resin may prevent it from flowing rapidly over the previous layer. Then, in preparation for curing the next layer, the previous layer is moved to the predetermined thickness of the working surface ensuring that the next layer will adhere to it after curing. This step of moving the previous layer away from the working surface by more than the predetermined thickness, and then moving it to the predetermined thickness, is typically referred to as over-dipping.
The liquid resins used are typically highly viscous, preferably having a viscosity as small as possible, which may be about 100 centipoise, and which typically ranges from between about 1000 to about 6000 centipoise.
As a result, during the recoating step, the resin may require an inordinate amount of time to flow over a previous layer and level out leaving an appropriate coating of resin over the previous layer in preparation for building the next layer, even where over-dipping is practiced. This may introduce distortions into the final part. This problem is particularly acute in the case where the stereolithographic part is comprised of many layers, since, in this instance, any delay in recoating would occur at each layer in the formation of the part, and therefore accumulate into a significant total delay. A third problem is that the resin may be so viscous and have too much surface tension that it will not settle into thin layers, which limits the final resolution of the part. With some current resin formulations, for example, layers thinner than about 5 mils may not be possible.
An approach which has been utilized to speed up the leveling of the liquid resin is the use of a blade known as a doctor or leveling blade to sweep over the resin surface, and scrape or push off excess resin to form a flat working surface. Blade recoating is, however, strongly dependant on part geometry. For certain geometries, for example, a particular blade speed may be too fast, so that too much resin is swept away by the blade, while for other geometries, this speed may be too slow to sweep away excess resin from the working surface. Instead, this resin may simply flow underneath the blade. Besides blade speed, other recoating parameters may have to be balanced depending on part geometry, including the number of passes made by the blade for a given layer, the speed of the blade at each pass, the part to blade clearance at each pass, the gap between the blade and the working surface, and the blade configuration.
It may be possible to recoat a given layer in a manner which is less dependent on part geometry. According to this approach, a given layer is recoated through more than one sweep of the doctor blade. However, such an approach can be time consuming in part because of the multiplicity of sweeps at each layer.
Another approach for forming a flat working surface may be the placement of a flat, rigid, and possibly transparent member in contact with the liquid resin such that the interface between the member and the resin forms the working surface for the formation of the next layer. This approach, however, is difficult to employ in the course of building up a multi-layer stereolithographic part, since the contact between the member and the resin may have to be broken periodically, typically for each layer of the part, to allow fresh liquid resin to flow over the working surface in preparation for curing additional layers. This can cause damage to the part.
In the case where the member is left in place while the resin at the working surface is cured to form a particular layer, the cured resin will bond to the member. When the member is removed from the cured resin so that fresh liquid resin may flow across the working surface in preparation for forming the next layer, the cured resin may stick to the member, and removal may be difficult even with the use of release agents. Moreover, cured resin adhering to the member may also cause the part to break, tear, or be dislocated as separation is attempted.
In the case where the member is removed before the resin at the working surface is cured to form a particular layer, liquid resin from the working surface may still stick to the member when it is removed, even with the use of release agents, thereby interfering with formation of a smooth working surface.
Accordingly, it is an object of the present invention to provide an apparatus and related method for rapidly providing a substantially flat working surface of photocurable resin for the formation of a multi-layer stereolithographic part, which apparatus and related method is substantially independent of part geometry, and which minimizes any distortive effects therefrom on the part.